Satellite positioning receivers determine their position by computing relative times of arrival of signals transmitted simultaneously from satellites such as the U.S. Global Positioning System (GPS) or NAVSTAR satellites. For example, the GPS Orbital Constellation has 28 satellites which orbit the earth in 12 hour orbits. The satellites are arranged in six orbital planes each containing four or more satellites. The orbital planes are spaced 60° apart from each other and are inclined approximately 55° with respect to the equatorial plane. This constellation provides a satellite positioning receiver with approximately four to twelve satellites visible from any point on earth. These satellites transmit, as part of their message, both satellite positioning data, so-called “ephemeris” data, as well as clock timing data. In addition, the satellites transmit time-of-week (TOW) information associated with the satellite signal, which allows the receiver to unambiguously determine time. The process of searching for and acquiring GPS signals, reading the ephemeris and other data for a multiplicity of satellites, and then computing the location of the receiver (and accurate time-of day) from this data is time consuming, sometimes requiring several minutes. In some applications, this lengthy processing time introduces unacceptable delays, and furthermore, reduces battery life in portable applications.
In addition, in many situations there may be blockage of the satellite signals. In these cases, the received signal level from the GPS satellites can be too low to demodulate and derive the satellite data signals without error. Such situations may arise in personal tracking and other highly mobile applications. In these situations, it may be difficult for a GPS receiver to acquire and track the GPS signals.